(NBC News) At least 13 people are dead following Monday morning's shooting spree at Washington, D.C.'s Navy Yard.
Several others were injured in the melee.
The gunman, identified as military contract worker Aaron Alexis, 34, is among the dead.
The U.S. Senate and other office buildings on the Capitol grounds have been locked down as a precaution.
Investigators are still looking into the possibility of a second gunman.
Early reports say as many as three shooters may have been involved, but one of those suspects has been identified and ruled out as a participant.
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